Field of the Invention
The present disclosure relates to a filtering circuit. More particularly, the present disclosure relates to a local area network (LAN) filtering circuit.
Description of Related Art
Common-mode choke (CMC) is typically used in switch power supply to reject unwanted common-mode signals (or called common mode noise) and thus electromagnetic emission generated by high frequency signal can be effectively suppressed. In fact, the common-mode choke is a bilateral filter, which not only rejects common-mode noise and signal but also suppresses electromagnetic emission from radiating into the environment to avoid affecting the other electronic products in the same electromagnetic environment.
Reference is made to FIG. 1, which shows a circuit diagram of a local area network (LAN) filtering circuit. The LAN filtering circuit includes a cable side 3 and a physical side 4, the cable side 3 is connected to a cable assembly, and the physical side 4 is connected to an external circuit board.
The LAN filtering circuit sandwiched between the cable side 3 and the physical side 4 includes a common-mode choke (CMC) 1 and an isolating transformer 2. The common-mode choke 1 shown in FIG. 1 is a 3-wire common mode choke. The common-mode choke 1 includes two opposite ends, wherein one end thereof includes a first input port 100, a second input port 120, and an input port 140 arranged in a sequential order, and the other end thereof includes a first output port 102, the second output port 122, and an output port 142 arranged in a sequential order.
The common-mode choke 1 further includes a first coil 10, a second coil 12, and a third coil 14. One terminal of the first coil 10 is electrically connected to the first input port 100, and the other terminal thereof is electrically connected to the first output port 102. One terminal of the second coil 12 is electrically connected to the second input port 120, and the other terminal thereof is electrically connected to the second output port 122. One terminal of the third coil 14 is electrically connected to the input port 140, and the other terminal thereof is electrically connected to the output port 142.
The isolating transformer 2 includes a first winding 20 and a second winding 22 coupling to each other. The first winding 20 has a first tap 202, a second tap 204, and a center tap 206. A distance between the first tap 202 and the second tap 204 is not only larger than a distance between the first tap 202 and the center tap 206 but also larger than a distance between second tap 204 and the center tap 206. The first tap 202 is connected to the first input port 100, the second tap 204 is connected to the input port 140, and the center tap 206 is connected to the second input port 120.
The second winding 22 includes a first tap 222, a second tap 224, and a center tap 226, wherein polarities of the first tap 222, the second tap 224, and the center tap 226 of the second winding 22 are the same as polarities of the first tap 202, the second tap 204, and the center tap 206 of the first winding 20. The first tap 222 of the second winding 22 is electrically connected to a high-level power rail (as symbol “+” shown in FIG. 1), the second tap 224 thereof is electrically connected to a low-level power rail (as symbol “−” shown in FIG. 1), and the center tap 226 thereof is electrically connected to a reference-level power rail (such as ground, and as symbol “G” shown in FIG. 1).
However, the connection manner of the common-mode choke 1 and the isolation transformer 2 shown in FIG. 1 makes a distance d1 (as shown in FIG. 2) between power rails for transmitting high-level signal (the first winding 10) and low-level signal (the third winding 14) is the largest distance between every two of the first winding 10, the second winding 12, and the third winding 14. Thus, the coupling coefficient of the local area network filtering circuit is poor, and a capability for resisting external noise is weak.
Hence, an improved local area network filtering circuit is required to overcome the above-mentioned disadvantages of the related art.